A BJT is a three-terminal electronic device constructed of two P-N junctions and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes. FIG. 1 shows an N-P-N BJT device 100 in accordance with the prior art. The BJT device 100 has three terminals: a collector C, a base B and an emitter E. Accordingly, the N-type BJT device 100 comprises an Nwell collector region 121, a Pwell base region 122 and a highly doped N-type emitter region 123. The base region 122 is located between the collector region 121 and the emitter region 123 and is surrounded by the collector region 121 making it almost impossible for the electrons injected into the base region 122 to escape being collected, thus giving the transistor a large gain.
In operation, the collector-emitter current ICE has a predetermined relationship with the base-emitter current IBE. In other words, ICE is controlled by IBE or controlled by the base-emitter voltage VBE. The proportion of ICE to IBE is generally referred to as the gain of the BJT device. Also, the BJT device 100 can have a breakdown voltage generally referred to as the collector-emitter breakdown voltage BVCEO when voltage is applied between the collector and the emitter with the base in open status. The breakdown voltage may also refer to the collector-base breakdown voltage BVCBO when a voltage is applied between the collector and the base with the emitter in open status.
It is believed that both the gain and the breakdown voltages are closely related to the vertical base width Lb, and to the lateral base width Ls. Since the control of the base width Ls laterally near the surface can be limited by the lithography technique, and due to the surface roughness, the bulk current path is preferred and Lb is controlled according to applications' requirements. At a given base doping concentration, when the vertical base width Lb is wide, the breakdown voltage is high and the gain is low. In practical applications, the vertical base width Lb needs to be controlled according to the specific requirements on the gain and/or the breakdown voltage. When the emitter region 123 has a predetermined size, the vertical base width Lb is believed to be determined by the base depth d1 of the base region 122.
For conventional well implantation approaches, the implanted well depth is controlled by precisely controlling the implantation conditions including controlling the ion-implantation dosage, energy, tilt and thermal annealing recipes. Since a different implantation condition is usually executed by applying an extra mask, additional mask is usually required for different well depths. Thus, if multiple BJT devices are integrated in a single semiconductor substrate with different parameters, or multiple Pwells or Nwells are fabricated in a semiconductor substrate requiring different implantation depths, multiple masks are required to define the specific depths. The multiple masks can add to the cost of manufacturing the semiconductor die. In addition, when the recipes for forming the BJT device including the Pwell, Pbase or Xbase are changed to improve the performances of other devices, the performance of a BJT device is also affected. Thus, extra mask(s) may be needed to prevent the performance of the BJT device being changed which further increases the cost of manufacturing.